skrunkwerks BMW R100 salt racer

The Salt!

BMW airhead rocker geometry

BMW airhead rocker geometry

Finally getting to the meaty end of the salt racer build.

While the crank is off getting balanced with the new pistons and rods, I had a look at the rocker geometry.

With the high lift cam (about 12mm at the valve) and recessed valve seats the geometry did not look good. With valve fully closed, the rocker nose was close to the edge of the valve stem end and moved just past the valve stem centerline at full lift. Not the best. Ideally, the nose travel across the stem end should be equidistant about the valve stem certerline.

The only way I could think to adjust this was to adjust the thickness of the spacers under the rocker posts. The stock item is 3mm. Replacing it with a 1.5mm spacer got things right as shown in the pic but only for that rocker! All the others were a bit different but acceptable with the 1.5mm spacer.

A stock pushrod was transmogrified into an adjustable one so that I could determine the correct length for the pushrods. I was going to have a shot at making my own tapered aluminium ones but just don’t have the time so it’s off to the pushrod man to have some made in 4130.


frame painted! – bike going back together

G Feeling like I’m on the home straight…frame is painted in special ‘skrunkwerks’ golden grey (stolen from Mercedes and tinkered with!).

Lots of werk fillet brazing and filling the various gussets to achieve a sculpted look. The engine case is painted in a 2 pack mat black while top engine cover and rocker covers painted in a 2 pack gloss.

skrunkwerks salt racer preview

H Here’s a sneak preview of  the salt racer before it comes apart to have the frame welding finished and painted. Also some detail shots of the top secret ‘ramstein’ airbox and frame.


BMW saltracer gets a carbon bum

II’m 6″ tall. With my chin on the tank, my arse ends up a couple of seconds behind! The seat will be quite long as a result and the rear-sets even further aft.

After playing around with pencil and paper getting proportions correct, I built a CAD 3D model, sliced it like a piece of virtual bread every 16mm, then printed and pasted these offsets onto 16mm customwood. Some jigsawing and glue created what looked like a wasps bum. When sanded smooth to the glue lines, the intended shape emerged.

My friends Peter and Steve of Applied Composite Technologies made a female mold from my ‘plug’ then they laid it up with carbon/epoxy to create the finished part. The pointy end was taken off and some curves introduced so that it looked the business on the salt racer.




Mal Hewett’s Vincent salt racer

S Self built, Vincent based, he’s scooped a couple records in the Vintage class at both Lake Gairdner and Bonneville. Love the man and his machine.

ABC did a short piece on him:

keeping the oil kule

Picture this: An air cooled 1000cc motor screaming it’s heads off at 2000 RPM past its stock red line in the heat of a parched salt lake in the middle of a desert.  And how about the head in the helmet? Need to avoid heat stroke in any of these three heads!

I’m using a Moteren Israel sump extension with a screw on oil filter. This is a very handy gadget as it has ports to add an external oil cooler. The sump extension is vital, not necessarily to increase oil quantity but to increase the sump volume to aid crank case breathing. The stock breather will also be enlarged.

Scored a hand-me-down oil cooler from the triumph salt racer that my shed mate Ross is building. A couple brackets and some plumbing later:

As for the head in the helmet…..







Airhead salt racer rear suspension geometry

Here’s a quick rundown of the process I went through in sorting the rear suspension geometry of the salt racer.

First task was to calculate the shock specifications. I started manually calculating these with the help of a couple texts including Tony Foale’s tome on the subject: Motorcycling Handling and Chassis Design. Even though the salt racer’s geometry is relatively simple compared to modern sports bikes with all their tricky linkages, I soon gave up and bought Tony’s program: Motorcycling Analysis which is basically the book in program form – this made things very much easier with the luxury of being able to easily experiment.  Highly recommended.

I’m not going to describe the process in detail but after inputting the geometry of the airhead frame + oilhead swingarm, I arrived at the following specifications for the shock (which ended up being similar to the stock R1100RT shock):

Free length: 340 (fully extended)
Travel: 50mm
Spring rate: 190N/mm
Lower clevis to suit the R1100 swing arm.

I opted for Hyperpro and soon after, a very sexy looking piece of Dutch bling landed on the doorstep complete with damping adjust both ways and ride hide adjust.

The program was used to predict the behavior of the system and tweek certain parameters so that ideal suspension characteristics were obtained – in this case we were shooting for a total sag (bike + Rider) of between 25-30mm with a spring preload of no more than 15mm. Most the program parameters as defined in the program’s set up page below are locked in:  The oilhead swing arm dimensions cannot be changed, nor can the spring specifications. The only parameters that can be changed are the ‘X coord’ and Y coord (which locate the top fixed end of the shock) and the spring preload. At this point we are mainly interested in the X coord and preload. The X coord is the horizontal mounting position of the shock in relation to the swing arm pivot.  I took an educated guess at the back wheel load at 112kg with me as rider.

A few button presses revealed that a X coord of 35mm combined with a spring preload of 13mm gave a total sag of 28mm. You can see the 35mm of X coord in the scaled graphic:  the top end of the shock is just behind the swing arm pivot. A spring preload of 13mm is a little on the high side for this particular spring. I could have reduced the necessary spring preload to get the target sag by pushing the top of the shock futher behind the swingarm pivot e.g. X coord of 40 or 50mm but there were other reasons why this was not so practical including complicating the mounting arrangement to the frame. So a X coord of 35mm was settled upon.   Now the Y coord could be tweaked, but this was done on the bike with shock attached to swingarm – more of this later.

Here’s the setup page of Tony’s program showing all the vital suspension stats of the salt racer. BTW, this is only the rear suspension part of the program. You could go on to model the entire suspension behavior of a bike if you so wished.

Tony Foale motorcycle setup page


The following is one of the many graphs that Tony’s program spits out. It shows shock displacement V’s wheel movement. The point to note is that from a completely unloaded state, the sag under full load (rider plus bike) is 28mm (vertical black line). Remember, nobody has sat on a bike yet, we are using the program to predict what the suspension will do in order to best locate the fixing point for the top end of the shock to the frame.




Another interesting graph is the effective spring rate measured at the wheel or ‘wheel rate’ Notice that even though the spring has a linear rate, the effective wheel rate is progressive ranging from approx 12 – 18 N/mm. This is the reason that people shouldn’t blindly ‘upgrade’ their linear springs to progressive rate springs. The suspension geometry may already be designed to obtain a progressive effective rate at the wheel with a linear spring. This is particularly the case for modern bikes with complex suspension linkages.




So,  having pegged the X coord at 35mm aft of the swing arm pivot, next job was to work out the vertical co-ordinate from the ground of the shock mounting position to the frame (Y coord). With a shaft drive, what we want to happen is this: when the bike is fully loaded, the gearbox output shaft axis, drive shaft axis and center of the rear wheel are all on the same straight line.  This ensures that any mechanical inefficiencies from universal joints operating at angles are dialed out – particularly for the salt racer where I’m chasing every microhorse. In reality, of course, this straight line position will become the median about which the suspension moves.

The image below shows the bike in an unloaded state (bike supported by jack under oil pan). The top edge of the bar with the blue tape marks the position where the drive train is perfectly aligned. With the top of the shock unattached, the bike was jacked up so that the center of the wheel was approx 25-28 mm below the top top edge of the bar. With top of shock 35mm aft of the swing arm pivot (Y coord) we now had the mounting position of the shock to the frame. I then mocked up a temporary mounting arrangement to lock the shock into this position. So, theoretically, with rider aboard the suspension should sag by the predicted 25-28mm at which point the drive train becomes aligned…




Now to test our predictions! With shock  firmly fixed to the frame I hopped on board. The measured total sag was 21mm. Not bad! I didn’t expect it to be spot-on because the bike was not complete and therefore under it’s finished weight. But it’s in a workable ball-park, there is enough adjustment in the system – adjustable torque arm and ride height adjust on the shock –  to tweak from this point so the next step is to remove the temporary bracketry and properly build in the top of the shock.  To be continued….






The business end

The frankenswingarm is finally looking like it should. Freshly powdercoated K100 17″ wheel; powdercoated R1100 RT swingarm; R1100S bevel drive; aftermarket carbon hugger for R1100S; Custom made adjustable torque arm. All grafted to the arse of an unsuspecting airhead.